1. Field of the Invention
This invention relates to organosiloxane compositions that cure by means of catalyzed reactions. More particularly, this invention relates to one-part organosiloxane compositions containing a novel microencapsulated curing catalyst. The compositions exhibit long term storage stability under ambient conditions yet cure rapidly at elevated temperatures.
2. Description of the Prior Art
Organosiloxane compositions cure by a variety of reactions. Some of the more common curing means involve 1) a free radical reaction initiated by the heat-induced decomposition of an organic peroxide or the decomposition of a photoinitiator in the presence of ultraviolet light and 2) the reaction between a hydroxyl-containing polyorganosiloxane and an organosilicon compound containing two or more hydrolyzable groups that occurs under ambient conditions in the presence of moisture and a suitable catalyst.
One of the more useful classes of polyorganosiloxane compositions cures by a reaction between silicon-bonded hydrogen atoms and either silicon bonded alkenyl radicals or hydroxyl groups. These reactions are catalyzed by metals from the platinum group of the periodic table or compounds of these metals. The advantages of these compositions include their rapid curing rate, particularly at elevated temperatures, the absence of objectionable by-products produced during the curing of compositions containing organic peroxides or silanes with hydrolyzable groups such as acetoxy or methoxy, and the difficulty of achieving complete curing of moisture-curable organosiloxane compositions applied in thick layers.
Compositions that cure by a hydrosilation reaction typically contain a polydiorganosiloxane with at least two ethylenically unsaturated hydrocarbon radical per molecule, an organohydrogensiloxane containing at least two silicon bonded hydrogen atoms per molecule in an amount sufficient to achieve curing of the composition and a platinum-or rhodium-containing catalyst in an amount sufficient to promote curing of the composition. Fillers and other additives may be present for the purpose of modifying physical and/or chemical properties of the composition either prior to or following curing.
Because organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction begin to cure even at ambient temperature once the reactants are combined, the catalyst and the organohydrogensiloxane reactant are usually packaged in separate containers and are not combined until it is desired to cure the composition. Even if the composition contains one or more of the known platinum catalyst inhibitors it cannot be stored in a single container for more than a few hours.
One of the alternatives proposed in the prior art to supplying platinum-catalyzed curable organosiloxane compositions as two-package materials is to isolate either the catalyst or the organohydrogensiloxane within a matrix of a material that is solid under the conditions encountered during storage of the curable compositions, yet allows the entrapped reactant or catalyst to escape and mix with the other ingredients when it is desired to cure the composition.
The prior art discloses a number of different one-part curable organosiloxane compositions containing microencapsulated reactants or catalysts. An example of this type of composition is described in U.S. Pat. No. 4,528,354, which issued to McDougal and Dougherty on July 9, 1985. This patent describes one-part peroxide curable silicone rubber compositions. The compositions include a microencapsulated liquid phase containing an organic peroxide in a shell of a resinous thermosetting material that is impervious to the peroxide.
The capsules are designed to rupture under a given internal vapor pressure that is generated-by-the encapsulated liquid when the curable composition containing the microcapsules is heated.
Because release of the peroxide is dependent upon rupturing rather than melting of the shell separating the peroxide from the other ingredients of the organosiloxane composition, the composition and thickness of the shell must be carefully controlled to ensure that the rupture of the capsules will occur reproducibly within the temperature range used to cure the organosiloxane composition.
U.S. Pat. No. 4,604,444, which issued to Donnadieu on Aug. 5, 1986 describes storage stable polyorganosiloxane compositions comprising a polyhydroxylated polyorganosiloxane, a polyfunctional acyloxysilane and a microencapsulated accelerator that either contains or generates water. The encapsulated material can be released using heat and/or irradiation. Suitable encapsulating materials include polystyrene, acrylonitrile-styrene copolymers, and poly(methyl methacrylate). This patent does not suggest using microencapsulated materials in organosiloxane compositions curable by means other than the reaction of polyhydroxylated polyorganosiloxanes with acyloxysilanes.
U.S. Pat. No. 4,461,854, which issued to Smith on July 24, 1984 teaches two-part curable organosiloxane compositions. One part contains a silanol-terminated polyorganosiloxane and the second component contains a curing agent, a filler and an encapsulated catalyst. The catalyst is a specified group of metal salts of carboxylic acids where the metal is, for example, tin, lead or zirconium. The encapsulating material is preferably a salt of a carboxylic acid that does not promote room temperature curing of the composition. The encapsulated catalyst prolongs the bath life of the curable composition.
U.S. Pat. No. 4,293,677, which issued to Imai on Oct. 6, 1981 describes encapsulating organohydrogensiloxanes using complex coacervation and in-situ polymerization, two of the most common microencapsulation techniques.
The in-situ polymerization process exemplified in example 2 of the Imai patent involves the polymerization of styrene in the presence of a dimethylsiloxane/methylhydrogensiloxane copolymer as the dispersed phase of an emulsion wherein the aqueous phase contains a solubilized polyvinyl alcohol and potassium persulfate.
A disadvantage of encapsulating the organohydrogensiloxane reactant as taught by Imai et al. is the relatively large amount of encapsulating polymer that is introduced into the composition. Many of the thermoplastic organic polymers suitable for use as encapsulants are incompatible with the reactants present in the curable composition. The presence of relatively large amounts of incompatible polymers may detract from the appearance, physical properties and optical properties of the cured material.
One way to reduce the amount of incompatible encapsulating polymer introduced into a curable composition is to encapsulate the platinum-containing catalyst rather than the organohydrogensiloxane reactant as taught by Imai et al. One of the most effective classes of catalysts for curing organosiloxane compositions of the type described in the Imai et al. patent are reaction products of an inorganic platinum compound such as chloroplatinic acid with liquid vinyl-containing organosilicon compounds such as sym-tetramethyldivinyldisiloxane. The solution can then be diluted to the desired platinum content, typically between 0.1 and 1 percent by weight, using a liquid dimethylvinylsiloxy terminated polydimethylsiloxane. Alternatively, the undiluted reaction product can be used as a catalyst.
U.S. Pat. No. 4,481,341, which issued to Schlak et al. on Nov. 6, 1984 and Japanese published application no. 49/134,786, published on Dec. 25, 1974 describe thermosetting organosiloxane compositions comprising a polyorganosiloxane containing at least two ethylenically unsaturated hydrocarbon radicals per molecule, a polyorganohydrogensiloxane containing at least two silicon bonded hydrogen atoms per molecule and a platinum-containing catalyst that is dispersed in a finely divided, solid matrix, such as a silicone resin or an organic resin. The concentration of catalyst is from 0.001 to 5 percent by weight of platinum metal.
The finely divided material in which the catalyst is dispersed is virtually insoluble in either the aforementioned polyorganosiloxane or polyorganohydrogensiloxane and melts or softens at a temperature between 70.degree. and 250.degree. C. The alleged advantage of the compositions disclosed in the patent to Schlak et al. is that the catalyst remains isolated from the other ingredients of the curable composition until the composition is heated sufficiently to melt the material in which the catalyst is dispersed. Because the organosilicon compounds present in the composition will not cure in the absence of the catalyst, the composition can allegedly be stored for long periods of time without undergoing curing or even an increase in viscosity.
A disadvantage of the curable organosiloxane compositions described by Schlak et al. and the published Japanese patent application is the method taught to prepare the catalyst/resin composition. A solid block or sheet of resin containing the platinum composition dispersed throughout is ground to a fine powder. Based on the random nature of the grinding operation there is a reasonable probability that some of the particles will contain platinum catalyst on their surface. Even trace amount of platinum have been shown to cause premature curing of the type of organosiloxane composition exemplified in this patent.
One way to avoid the inherent disadvantages of the catalyst compositions described in the Schlak et al. patent, is to completely microencapsulate finely divided particles or droplets of a catalyst composition within a material that is impermeable to the catalyst and effectively isolates it from the reactive ingredients of a curable organosiloxane composition. The encapsulant melts or softens at the desired curing temperature of the composition. A variety of methods for microencapsulating materials are known in the art.
U.S. Pat. No. 4,874,667, which issued on Oct. 17, 1989 to Lee et al. and is assigned to the same party as the present application discloses one-part organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction. The platinum catalyst is microencapsulated in one or two layers of a thermoplastic organic polymer. The diameter of the microencapsulated catalyst particles are less then 100 microns.
A disadvantage of the preparative methods described in the Lee et al. patent and related U.S. Pat. No. 4,766,176, issued on Aug. 23, 1988 and U.S. Pat. No. 4,784,879, issued on Nov. 15, 1988, is the inability of these methods to yield microcapsules of sufficiently small size that curable compositions containing these microcapsules are optically transparent. Curable compositions containing these microcapsules are translucent or opaque.
U.S. patent application Ser. No. 07/431,352, filed on Nov. 3, 1989 and assigned to the same party as the present application, is directed to heat curable silicone compositions useful for application to paper as release coatings. The compositions comprise (1) an organosilicon compound containing a plurality of silicon bonded hydroxyl groups and/or alkenyl radicals, (2) an organohydrogensiloxane containing an average of at least two silicon-bonded hydrogen atoms per molecule, (3) a platinum group metal-containing catalyst in an amount sufficient to accelerate curing of the composition and (4) a platinum catalyst inhibitor in an amount sufficient to retard the curing reaction at room temperature but insufficient to prevent the reaction at elevated temperature. The composition is typically in the form of a coating bath. A continuous length of paper or other material to be coated is drawn from a feed roll, passed through the coating bath and cured prior to being wound on to a take-up roll.
The inventive ingredient in the composition is a bath life extender which itself is ineffective in prolonging the useful life of the curable composition, but when used in combination with the platinum catalyst inhibitor extends the useful life of the composition at room temperature without substantially lengthening the time required to cure the composition at the typical curing temperatures of from about 70.degree. to 120.degree. C.
Useful bath life extenders are defined in terms of their Hansen partial solubility parameter for hydrogen bonding. This parameter is greater than 8.0 for useful bath life extenders, a preferred range being from 13 to 48. Preferred bath life extenders are organic compounds containing one or more primary and/or secondary alcoholic hydroxyl groups, carboxylic acids, cyclic ethers and water. Primary and secondary alcohols are particularly preferred bath life extenders. Bath lives of greater than 168 hours are reported for the combination of diethyl fumarate as the catalyst inhibitor and benzyl alcohol as the bath life extender.
The present inventors modified the composition described in the aforementioned application Ser. No. 07/431,352 by replacing the bath life extender with a hydroxy-substituted ester of a polymerizable ethylenically unsaturated acid. When the ester was subsequently polymerized the resultant composition exhibited a surprising increase in long-term storage stability relative to compositions containing the bath life extenders described in the patent application. Examination of the composition under an electron microscope revealed the platinum group metal catalyst in the form of discrete microcapsules exhibiting diameters of less than three microns.
The present inventors believe that the same method used by them to extend the storage life of curable organosiloxane compositions containing platinum group metal-containing catalysts is applicable to other 1-part organosiloxane compositions whose cure rate is accelerated in the presence of metal-containing catalysts that form complexes with ethylenically or acetylenically unsaturated organic compounds.
One objective of this invention is to provide a novel type of microencapsulated platinum group metal-containing curing catalyst that does not detract from the optical transparency or the cure rate at elevated temperatures of curable organosiloxane compositions containing the catalyst.
A second objective is to provide a method for preparing microencapsulated catalysts containing compounds of platinum group metals and compounds of other metals that are effective curing catalysts for organosiloxane compositions.
Another objective is to provide optically transparent one-part storage stable organosiloxane compositions containing the microencapsulated curing catalysts of this invention.